1. Field of the Invention
The present invention relates to a semiconductor laser module, and an optical amplifier using the semiconductor laser module, and more particularly, to a semiconductor laser device provided with two stripes from which two laser beams are emitted, and an optical amplifier.
2. Discussion of the Background
With progress in optical communications based on a high-density wavelength division multiplexing transmission system over the recent years, a higher output is increasingly demanded to a pumping light source used for the optical amplifier.
Further, a greater expectation is recently given to a Raman amplifier as means for amplifying the beams having a much broader band than by an erbium-doped optical amplifier that has hitherto been used as the optical amplifier. The Raman amplification is defined as a method of amplifying the optical signals, which utilizes such a phenomenon that a gain occurs on the side of frequencies as low as about 13 THz from a pumping beam wavelength due to the stimulated Raman scattering occurred when the pumping beams enter an optical fiber, and, when signal beams having the wavelength band containing the gain described above are inputted to the optical fiber in the thus pumped (excited) state, these signal beams are amplified.
According to the Raman amplification, the signal beams are amplified in a state where a polarizing direction of the signal beams is coincident with a polarizing direction of the pumping beams, and it is therefore required that an influence caused by a deviation between polarizing planes of the signal beam and of the pumping beam be minimized. For attaining this, a degree of polarization (DOP) has hitherto been reduced by obviating the polarization of the pumping beam (depolarization).
As a method for simultaneously realizing a higher output and depolarization of a pumping light source, as disclosed in U.S. Pat. No. 5,589,684, a method in which a laser beam emitted from two semiconductor laser modules oscillating on the same wavelength is polarization-synthesized by a polarization synthesizing coupler is known.
FIG. 35 is an explanatory view in explaining a conventional semiconductor laser apparatus disclosed in U.S. Pat. No. 5,589,684.
As shown in FIG. 35, a conventional semiconductor laser apparatus comprises: a first semiconductor laser device 100 and a second semiconductor laser device 101 for emitting laser beams in the orthogonal direction with each other on the same wavelength; a first collimation lens 102 for collimating the laser beam emitted from the first semiconductor laser device 100; a second collimation lens 103 for collimating the laser beam emitted from the second semiconductor laser device 101; a polarization synthesizing coupler 104 for orthogonally polarization-synthesizing the laser beam collimated by the first collimation lens 102 and the second collimation lens 103; a condenser lens 105 for condensing the laser beams polarization-synthesized by the polarization synthesizing coupler 104; and an optical fiber 107 with a fiber Bragg grating (FBG) 106 for receiving the laser beams condensed by the condenser lens 105 and letting the beams travel to the outside.
According to a conventional semiconductor laser apparatus, since the laser beams emitted in the orthogonal direction with each other from the first semiconductor laser device 100 and the second semiconductor laser device 101 are polarization-synthesized by the polarization synthesizing coupler 104, a laser beam whose degree of polarization is small can be emitted from the optical fiber 107. Furthermore, since fiber Bragg grating 106 is formed in the optical fiber 107, oscillation wavelengths of the semiconductor laser devices 100 and 101 are fixed in the same degree, a laser beam whose wavelength is fixed can be emitted from the optical fiber 107.
Accordingly, the above-mentioned conventional semiconductor laser apparatus can be applied as a pumping light source of an optical amplifier which requires a high optical output, especially of a Raman amplifier, which requires a low polarization dependency and a wavelength stability.
A conventional semiconductor laser apparatus has the following problems.
(1) In the conventional semiconductor laser apparatus, two chip carriers with two semiconductor laser devices 100 and 101 attached thereto respectively need to be disposed on a base plate by soldering. At this time, since positioning need to be conducted so that laser beams emitted from the two semiconductor laser devices 100 and 101 be orthogonal with each other, it is difficult to conduct the positioning of the semiconductor laser devices and a time for positioning becomes longer. As a result, a time for manufacturing a semiconductor laser module is increased.
(2) Since the beams emitted from each of the semiconductor laser devices 100 and 101 are emitted in completely different directions from each other, there arises, for example, a warp of a package in which optical components are aligned and fixed under a state of a high temperature. Due to this, it is difficult to stabilize beam intensity and a degree of polarization of the beam emitted from the optical fiber.
(3) In the conventional semiconductor laser apparatus, since collimation lenses 102 and 103 for collimating the laser beams emitted from the semiconductor laser device 100 are used, a beam diameter and an image magnification are enlarged. Therefore, there is a problem in that a tolerance for the position and angle is strict.
(4) In order to cool the two semiconductor laser devices 100 and 101 positioned at a space, a large-sized Peltier module is required. As a result, there is a problem in that the electric power consumption of a semiconductor laser module is increased.
(5) In the conventional semiconductor laser apparatus, an optical fiber with the fiber Bragg grating 106 and the semiconductor laser devices 100 and 101 need to be optically coupled. Since the optical coupling is performed mechanically in a resonator, there is a fear that an oscillation characteristic of the laser beam is changed due to a mechanical vibration. Therefore, there is a problem in that it is impossible to provide a stable optical output in some cases.
(6) Wavelengths of the laser beams emitted from each of the semiconductor laser devices 100 and 101 are determined by a sole FBG and thus a degree of freedom for setting up the wavelengths of each of the stripes is not provided.
Furthermore, when this semiconductor laser device is regarded as a pumping light source used for the Raman amplification, there are the following problems.
(7) In the conventional semiconductor laser device, since the semiconductor laser devices 100 and 101 and the fiber Bragg grating 106 are widely spaced, relative intensity noise (RIN) is made loud due to resonance between the fiber Bragg grating 106 and an optical reflection surface. For example, since amplification occurs at an early stage in the Raman amplification, if a pumping beam intensity fluctuates, a Raman gain also fluctuates. Therefore, fluctuation of the Raman gain is outputted as fluctuation of an amplified signal intensity as it is and there is a problem in that a stable Raman amplification can not be conducted.
(8) As an optical amplification method, in addition to a back pumping method in which a pumping beam is supplied in the opposite direction of the traveling direction of the signal beam, there is a forward pumping method in which a pumping beam is supplied in the same direction of the traveling direction of the signal beam and a bidirectional pumping method in which pumping is conducted from both the directions. At present, the back pumping method is mainly used as the Raman amplifier. This is because in the forward pumping method in which a weak signal beam moves together with a strong pumping beam in the same direction, there is a problem in that the fluctuation of the pumping beam intensity largely influences on the fluctuation of the amplified signal beam. Therefore, a stable pumping light source which can be applied to the forward pumping method is demanded. In other words, when the semiconductor laser module using a conventional fiber Bragg grading is employed, there is a problem in that applicable pumping methods are limited.
(9) In the Raman amplification in the Raman amplifier, it is defined as a condition that the polarizing direction of the signal beams is coincident with the polarizing direction of the pumping beams. That is, in the Raman amplification, there is a polarization dependency of an amplification gain, and an influence due to a deviation between the polarizing direction of the signal beam and the polarizing direction of the pumping beam should be minimized. Here, in the case of the back pumping method, since polarization becomes random during propagation in the signal beam, there arises no problem. However, in the case of the forward pumping method, polarization dependency is strong and thus the polarization dependency needs to be reduced by orthogonal polarization synthesizing of the pumping beam, depolarization and the like.
In other words, a degree of polarization (DOP) needs to be minimized. Furthermore, since in Raman amplification, the obtained amplification ratio is relatively low, a pumping light source for the Raman amplification having a high output has been demanded.
In contrast to the prior art, the embodiments of the present invention are for a semiconductor laser module, a manufacturing method thereof and an optical amplifier that are capable of obtaining a high optical coupling efficiency, attaining a down-size and a mass-production, and reducing both of a manufacturing time and a manufacturing cost.
The present invention provides a semiconductor laser device having a plurality of stripes formed at a space, wherein laser beams are emitted from one-sided edge surfaces of the respective stripes and diffraction gratings are provided in the respective stripes.
In particular, the present invention provides a semiconductor laser device comprising a first light emitting stripe aligned to emit a first laser beam through one edge surface and at least one other light emitting stripe aligned to emit at least one other laser beam through the one edge surface, wherein the first light emitting stripe aligned to emit the first laser beam through the one edge surface and an opposite edge surface and the least one other light emitting stripe aligned to emit at the least one other laser beam through the one edge surface and the opposite edge surface.